[msbuild] Use the right trailing slash for _AppContainerDir according to the current platform.

msbuild/Xamarin.Shared/Xamarin.Shared.targets

@@ -2759,7 +2759,7 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 			<AppBundleDir>$(_AppContainerDir)$(_AppBundleName)$(AppBundleExtension)</AppBundleDir>
 		</PropertyGroup>
 		<PropertyGroup Condition="'$(_AppContainerDir)' == ''">
-			 <_AppContainerDir>$([System.IO.Path]::GetDirectoryName($(AppBundleDir)))/</_AppContainerDir>
+			 <_AppContainerDir>$([MSBuild]::EnsureTrailingSlash($([System.IO.Path]::GetDirectoryName($(AppBundleDir)))))</_AppContainerDir>
 		</PropertyGroup>
 		<PropertyGroup>
 			<!-- Ensure _AppBundlePath is a relative path (relative to the project directory) and contains a trailing slash -->